Blood coagulation prevents hemorrhage and also contributes to thrombotic diseases including myocardial infarction, stroke, and venous thromboembolism. A great deal has been learned about blood coagulation on the scale of a test tube, an artery, and a vein. However, the regulation of blood coagulation at the single cell level has been largely unexplored. Under ordinary conditions endothelial cells that line all arteries and veins have anticoagulant properties. However, following injury or stress, endothelial cells develop some procoagulant properties and recruit platelets that respond with promounced procoagulant activity. Our preliminary data demonstrates that stressed or stimulated endothelial cells support assembly of the prothrombinase complex in focal areas on the cell while maintaining anticoagulant proteins on other parts of the cell. They identify TMEM16F as a protein required for normal platlet procoagulant activity. Localization of procoagulant activity is dependent upon regulated phosphatidylserine exposure and convex curvature of membrane regions. Thus our data suggest that stressed, viable endothelial cells can support procoagulant enzyme activity at the subcellular level, assisted by platelets, while maintaining anticoagulant properties. The objectives of this proposal are to characterize the role of TMEM16F with regard to procoagulant activity of platelets and endothelial cells using engineered mice lacking the protein. We will explore the functional consequences of the localized enzyme complexes on endothelial cells and determine whether TMEM16F- mediated phosphatidylserine exposure is also critical there. Finally, we will apply the insights gained to test a novel hypothesis in which focal fibrin deposition, mediated by endothelial cell stimulation, leads to trapping and elimination of many types of bacteria. The results of our studies are relevant to fundamental understanding of the blood coagulation mechanism and its participation in heart attacks and strokes. It is also related to the role of the blood coagulation mechanism in preventing or limiting bacterial infections. Finally, it may also help to explain the relationship of blood coagulation to inflammatory diseases.